The present invention relates generally to a storage device using a dynamic RAM. More particularly, the present invention relates to a storage device that preserves the contents of a dynamic RAM by performing a self-refreshing operation when a main power source is shut off, and the storage device operates under a backup power supply.
In conventional storage devices using a dynamic RAM, refreshing is performed during a normal operation so as to maintain the contents of the dynamic RAM. In the refreshing operation, a refresh instruction signal is given by an upper level device to the storage device, and this instruction is interpreted by a memory controller. According to the interpretation result, the memory controller changes in turn CAS (column address strobe) and RAS (row address strobe) signals acting as dynamic RAM control signals to an L-level with predetermined timing. This refreshing operation is known as the CAS-before-RAS refreshing operation.
The refreshing operation usually needs a rather large current such as a few ten mA. When the main power source to the storage device is shut off, backup power is supplied to the dynamic RAM from a backup power source such as a battery, and the data stored in the dynamic RAM is preserved by the self-refreshing capability of the dynamic RAM while consuming as small electric power as static RAMS. In the power backup mode, unlike the normal mode, the self-refreshing for preserving the data in the dynamic RAM is performed in such a manner that the CAS and RAS signals applied to the dynamic RAM are changed in turn to an L-level with predetermined timing and maintained at the L-level for a time period equal to or greater than a predetermined value so that a refreshing circuit in the dynamic RAM starts its operation by which memory cells are refreshed automatically. Both RAS and CAS signals are fixed to the L-level during the refreshing operation.
In this situation, the self-refreshing operation of the dynamic RAM needs a current as low as a few hundred .mu.A, which allows a long-time operation in the power backup mode. The RAS and CAS signal lines have to be maintained firmly at an L-level so that the self-refreshing operation can be performed correctly even at the moment when the main power source of the device is turned on or off. To meet this requirement, in the conventional technique, the backup power is supplied to the whole memory circuit including the controller.
However, unlike the static RAM that can maintain data with very low power consumption, the conventional storage device using a dynamic RAM consume considerable power when the dynamic RAM is refreshed to maintain data in the dynamic RAM, and thus a high-capacity battery is required to achieve a sufficiently long backup. Furthermore, in the conventional technique, the controlling circuit also consumes electric power during the backup operation, which further limits the maximum backup time.
In storage devices of the type in which expansion of the dynamic RAM memory modules is allowed up to a predetermined upper limit, an additional memory module for expansion is installed when the storage device is in a backup mode in which the main power source to the storage device is shut off. However, the memory controlling circuit of the newly installed memory module does not execute a sequence of operations required to start a self-refreshing operation when the backup power source starts to supply electric power to the new memory module. As a result, a newly installed memory module consumes as great electric power in the backup-mode operation as in a normal-mode operation.
In the normal operation mode, the dynamic RAM needs power consumption a few thousand times greater than the power consumption required when the operation is performed in the self-refreshing mode. Therefore, the storage device having expanded memory modules runs out of battery energy in a very short time. Even if the RAS and CAS signals, produced by the controlling circuits of the newly installed memory module are lowered in turn to the L-level so as to start a self-refreshing operation, the new memory module consumes rather great electric power during a time period from the installation of the memory module to the start of the self-refreshing operation.